Questions on old 1964 2300V 350 hp cooling tower motor efficiency

[razorbackengineer]

Hey guys, just joined this forum. Thought I'd give this a try.

Trying to determine if there's any value in replacing an old cooling tower motor at our plant with a more efficient one. New mechanical engineer here, been doing quite a bit of research on these 3-phase motors (my knowledge base is from a couple semesters of circuits and electronics in college), so forgive the ignorance. If anyone could answer any of these questions, it would be helpful. The motor manufacturer has gone out of business.

1. The motor is old enough that there's not even a nominal efficiency rating on the nameplate. The only nameplate info is "Allis Chalmers 350 hp 2300V 86A 60Hz 1800 rpm 3ph". That's it. Any way to determine nameplate efficiency?

2. I've been told that because it's a 2300V motor, it has a form-wound stator. From my research, that seems true. One guy from a motor shop we use said the efficiency of form-wound stator motors has changed little over the years, from about 94.6% efficiency to 95.9%. Does this seem accurate?

3. Any word on efficiency differences between form-wound and random-wound stators? (Just a general curiosity from my reading I couldn't answer)

4. It seems like it's not going to be worth upgrading the motor to a high-efficiency one until this one craps out, based on all the reading I've done. Any other considerations I should consider in recommending we just wait?


Thanks!

 

[ScottyUK]

Cooling tower duty is among the worst imaginable places to put anything electrical: hot, wet, chemical-laden, and corrosive due to slight excess of either hypo or sulphuric acid present at some level. If you have a motor which has stood up to that environment for 50 years then definitely don't swap it for anything modern. Even good brands struggle in that environment. Frankly that must be an exceptionally well-built motor.

I think I'd look for easier efficiency improvements - the savings from a 1% gain in efficiency of a relatively small plant auxiliary drive could easily be wiped out by poorer availability and increased maintenance costs. What type of station are you operating? Big coal plants are often horribly inefficient but brutally simple and achieve admirable availability and reliability. Figure out the cost of downtime in each place that you look at replacing equipment on the grounds of efficiency, because a lot of new equipment doesn't have the huge margins the old gear had to shrug off abuse, overloading, and environmental conditions.

 

[razorbackengineer]

Thanks for the feedback.

Interesting. Yeah these are cooling towers for process cooling at a chemical plant. They are external to the towers though. I have heard some folks mention the "not built like they used to" mantra a few times with reference to our old motors, but I didn't know if that was really the case. But yeah all those large motors for the pumps are very old. I was having trouble finding any repair history on these motors at our plant too, so either our paperkeeping really is that poor or we've had to repair the motors very few times. I just figured, without knowing anything, a 50 year old motor would be terribly inefficient and there'd be value in replacing it. Maybe not.

If anyone has any more input on those efficiency questions I raised as well, I'd be all ears.

 

[jraef]

To further ScottyUKs point, attaining that theoretical 1% gain in efficiency on a motor that size is unlikely to begin with. And yes, your motor shop guy is probably correct, 94% eff or greater even on an older form wound motor would be very common on something that size. The NEMA definition of "Premium Efficieny" motors over 600V is 94.1% minimum, 95% nominal, regardless of enclosure, number of poles etc. I would not fix something that is not broken just to try to potentially get a tiny incremental gain in efficiency.

That said, if you are going to add a VFD to it to save energy and the choices include a Voltage Source Inverter, you may want to consider changing the motor prophylactically. No need if it is a modern Current Source Inverter though.






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